The present invention relates to a control system for a working machine, comprising a device for changing direction of the working machine, an operator controlled element and an electronic control unit adapted to receive an input signal indicative of desired direction from the position of the operator controlled element and to generate an output signal for producing a change of direction of the working machine, corresponding to the position of the operator controlled element, via said device. The operator controlled element is preferably constituted of a steering lever.
Such a control system is found, for example, in different types of articulated working machines comprising a front and a rear frame, said frames each carrying at least one wheel axle. The frames are articulately connected to each other via a vertical axis and the frames are pivotally mounted relative to each other about said joint via two hydraulic cylinders arranged between the two sections. A loading machine, such as a wheel loader or a dumper, is an example of such a working machine.
Below, the invention will be described for the case when the control system is arranged in a wheel loader for controlling its steering cylinders. This is to be regarded as a by no means limiting example within the field of application of the invention.
Traditionally, the device for changing direction of the working machine comprises a hyDraulic system adapted to control the movement of the hydraulic cylinders via control valves.
The steering in a wheel loader has high security requirements. As a rule, a wheel loader has a conventional steering wheel control, but can in parallel also be equipped with a lever control. The steering wheel is used during transport, and the lever is used for work at lower speeds such as, for example, when loading a dumper or truck, this in order to increase the operator's comfort. Accordingly, the operator uses the steering lever to request steering direction and steering speed.
The control system is usually designed such that the operator, by means of the steering wheel control, mechanically regulates a hydraulic valve which, in its turn, transfers hydraulic power to the steering cylinders. The lever control also transfers hydraulic power to the steering cylinders, but its hydraulic valve is normally controlled by electrical signals, that is to say the operator tilts the lever, which in its turn generates an electrical signal. In certain cases, it is desired to manipulate the electrical signal from the lever, which can be done by transferring the signals from lever to hydraulic valve via the control unit (machine computer). Examples of this are ramp elements (filters) and a steering speed which is depending on the speed of the machine, with the purpose of obtaining a smooth control in a loading operation, but still a directionally stable (non-wobbling) steering during transport.
In order to increase the safety, the control system can be designed such that the steering wheel control has the highest priority, that is to say, if the steering wheel and the lever are used at the same time, it is the steering wheel which is dominant.
Higher and higher demands are placed on the electronics for safety critical functions, such as the lever control, in the machine. The lever control with its electronics is a complex system with many sources of malfunction. A particular problem is that malfunctions can occur in the lever control (for example that steering takes place even if the lever is in a neutral position or the lever has locked in a certain position). Therefore, very high quality demands are placed on the machine computer and other electronics, which results in high costs.
It is desirable to produce a control system which offers a high safety in a cost efficient way. In particular, it is desirable to provide a control system which remedies the problem when malfunctions occur in the lever control (for example that steering takes place even if the lever is in a neutral position).
According to an aspect of the present invention, a control system comprises an electronic safety device adapted to be controlled by the position of the operator controlled element and adapted to allow control of the device via said output signal, and to block control of the device via said output signal, respectively, depending on the position of the operator controlled element.
According to one embodiment of the invention, the safety device comprises a control assembly arranged between the control unit and said device for changing direction in such a way that an output signal generated by the control unit is only allowed to pass when the output signal corresponds to the position of the operator controlled element.
The control assembly (for example comprising two relays) is then arranged on the lines for transferring the output signals to the device (for example in the form of a hydraulic circuit). By means of automatically opening and closing, respectively, the lines depending on the lever position, a safe system is obtained.
According to another embodiment, said electronic safety device comprises a means adapted to sense a first angular interval of the operator controlled element, said angular interval being associated with a first steering direction, and a second angular interval of the operator controlled element, said angular interval being associated with a second steering direction. In the safety device, the position of the operator controlled element is preferably sensed fully independently of the input signal to the control unit from the position of the operator controlled element. This means that the safety assembly becomes a parallel, independent system, relative to the traditional lever control. Preferably, said means for sensing said angular interval of the operator controlled element comprises a first switch associated with the first angular interval of the operator controlled element and a second switch associated with the second angular interval of the operator controlled element.
Further preferred embodiments and advantages of the invention are apparent from the following description.